This invention relates to a process and apparatus for the treatment of hydrocarbon oils and, more particularly, to the hydroconversion of heavy hydrocarbon oils in the presence of particulate additives, e.g. iron and/or coal additives.
Hydroconversion processes for the conversion of heavy hydrocarbon oils to light and intermediate naphthas of good quality for reforming feedstocks, fuel oil and gas oil are well known. These heavy hydrocarbon oils can be such materials as petroleum crude oil, atmospheric tar bottoms products, vacuum tar bottoms products, heavy cycle oils, shale oils, coal derived liquids, crude oil residuum, topped crude oils and the heavy bituminous oils extracted from oil sands. Of particular interest are the oils extracted from oil sands and which contain wide boiling range materials from naphthas through kerosene, gas oil, pitch, etc., and which contain a large portion of material boiling above 524xc2x0 C. equivalent atmospheric boiling point.
As the reserves of conventional crude oils decline, these heavy oils must be upgraded to meet the demand for lighter products. In this upgrading, the heavier materials are converted to lighter fractions and most of the sulphur, nitrogen and metals must be removed.
This can be done either by a coking process, such as delayed or fluidized coking, or by a hydrogen addition process such as thermal or catalytic hydrocracking. The distillate yield from the coking process is typically about 80 wt % and this process also yields substantial amounts of coke as by-product.
Work has also been done on a new processing route involving hydrogen addition at high pressures and temperatures and this has been found to be quite promising. In this process, hydrogen and heavy oil are pumped upwardly through an empty tubular reactor in the absence of any catalyst. It has been found that the high molecular weight compounds hydrogenate and/or hydrocrack into lower boiling materials. Simultaneous desulphurization, demetallization and denitrogenation reactions take place.
Additives have been developed which can suppress coking reactions or can remove the coke from the reactor. It has been shown in Khulbe et al, U.S. Pat. No. 4,923,838 issued May 8, 1990 that the formation of carbonaceous deposits in the reaction zone can be substantially reduced by mixing with a heavy oil feedstock a finely divided particulate consisting of carbonaceous particles and particles of an iron compound, e.g. an iron salt or oxide such as iron sulphate. The particles typically have average sizes of less than 10 xcexcm. Canadian Patent No. 1,202,588 describes a process for hydrocracking heavy oils in the presence of an additive in the form of a dry mixture of coal and iron salt, such as iron sulphate.
A problem in the hydroprocessing of heavy hydrocarbon oil containing finely divided particulate, such as iron sulphate, is to achieve a good gas-liquid distribution in a reaction zone while avoiding coke formation and build-up. Bubble cap distribution plates are commonly used for gas-liquid distribution, e.g. as described in U.S. Pat. 4,874,583 issued Oct. 17, 1989, etc. However, when all gas, liquid and particulate are introduced into a lower region of a reaction zone below a bubble cap distribution plate, there is a problem that the bubble caps are quickly plugged and flow is reduced.
It is the object of the present invention to provide improvements to the mixing of hot hydrogen containing gas with heavy hydrocarbon oil in a hydrocracker and to ensure that additive particles are well mixed into the reactor contents and no settling occurs in the bottom head of the reactor.
According to the present invention, it has been discovered that further improvements in the hydroprocessing of heavy hydrocarbon oils containing additive particles to suppress coke formation are achieved by the manner in which the heavy hydrocarbon oil and additive particles are introduced into the bottom of a reactor and the manner in which hot hydrogen-containing gas is introduced into the mixture of heavy hydrocarbon oil and additive particles within the reactor.
Thus, one embodiment of the present invention in its broadest aspect relates to a process for hydrocracking a heavy hydrocarbon oil in which (a) a slurry feed comprising a mixture of a heavy hydrocarbon feedstock and from about 0.04 to 4.0% by weight (based on fresh feedstock) of coke-inhibiting additive particles having an average particle size of less than about 30 xcexcm, preferably less than about 10 xcexcm, and (b) a hydrogen-containing gas, are passed upwardly through a confined hydrocracking zone in a vertical, elongated, cylindrical vessel with a generally dome-shaped bottom head. The hydrocracking zone is maintained at a temperature between about 350xc2x0 C. and 600xc2x0 C. and a pressure of at least about 3.5 MPa. From the top of the hydrocracking zone there is removed a mixed effluent containing a gaseous phase comprising hydrogen and vaporous hydrocarbons and a liquid phase containing heavy hydrocarbons and particulates.
According to the novel features of this process, the slurry feed mixture and a portion of the hydrogen-containing gas (secondary gas) are fed into the hydrocracking zone through a feed injector at the bottom of the dome-shaped bottom head. The balance of the hydrogen-containing gas (main gas) is fed into the hydrocracking zone through a plurality of injection nozzles in the hydrocracking zone at a location above the slurry-feed injector. The temperature of the main hydrogen-containing gas entering through the nozzles is higher than the temperature of the combined slurry feed and hydrogen-containing gas entering through the bottom feed injector, and is generally sufficient to maintain the contents of the hydrocracking zone at a desired operating temperature. The main gas temperature is typically in the range of about 450 to 600xc2x0 C., preferably about 450 to 540xc2x0 C. The combined slurry feed and secondary gas entering through the bottom feed injector should enter at a velocity of at least 5 m/s whereby the additive particles are maintained in suspension throughout the reactor vessel and coking reactions are prevented. The combined slurry feed and secondary gas enters the reactor typically at a temperature in the range of about 300 to 430xc2x0 C., preferably about 350 to 390xc2x0 C. In a typical process according to the invention, the temperature of the vessel contents varies between about 440xc2x0 C. in a lower region and 465xc2x0 C. in an upper region.
The hydrogen-containing gas preferably comprises a recycle gas stream rich in hydrogen typically containing at least 60% hydrogen, and an important feature of this invention is the manner in which this hydrogen gas is introduced into the hydrocracking zone. In order to achieve a good contact between the main recycle hydrogen stream and the heavy hydrocarbon oil in the hydrocracking zone, it is important that the main recycle hydrogen stream be uniformly distributed within the hydrocracking zone in the form of high velocity jets, which provide high shear and mixing, producing small bubbles, to give large surface area for mass transfer from the hydrogen in the bubbles to the bulk liquid above the distributor.
In order to achieve these results, the main gas is preferably injected into the hydrocracking zone through injection nozzles that are arranged to assist in the uniform distribution of the content of the hydrocracking zone. The slurry feed and gas fed in through the bottom feed injector tends to create some central channelling of the flow within the hydrocracking zone. Thus, there is a tendency for much of the gas to flow up the middle of the reactor, with liquid and particulate flowing down the sides. It is, therefore, preferable to provide a lower central set of gas injector nozzles so that the gas flowing from the nozzles is adapted to disperse the central channelling in an outward direction toward the vessel walls. These lower nozzles are preferably arranged in a central circle with the nozzles aimed in an upward and outward direction.
It is also preferable to provide a second set of gas injection nozzles in the vessel at a location above the lower nozzles. These second nozzles are arranged adjacent the wall of the vessel, with the majority being aimed in an upward direction. Some of these second nozzles are also aimed in a downward and inward direction. The upwardly directed second nozzles serve to inhibit flow of liquid and particulates down the vessel walls.
The individual nozzles are typically in the form of small tubes, preferably having an inner diameter of about 6 to 25 mm and lengths of about 50 to 100 mm. The pressure drop in these nozzles is preferably quite substantial, e.g. in the order of 30% of the liquid head in the vessels, and they can be operated at quite high velocities, e.g. in the order of at least 120 m/sec. and as high as 200 m/sec. Such high velocities provide sufficient kinetic energy to cause attrition of the particulate in the vessel. Attrition depends on the square of the velocity and the upper limit of 200 m/sec. is to limit the production of very fine powder. These high velocities do not appear to cause foaming within the vessel.
It is also possible to arrange the nozzles all at the same level and equally spaced across the vessel. In this arrangement, all of the nozzles are in the form of small vertical tubes aimed upwardly. While this arrangement gives a relatively flat gas profile across the hydrocracking zone, there remains some tendency for the gas to channel upwardly in the central region and liquid and particulate to flow down the walls.
Another important feature of this invention is that a portion of the hydrogen-containing gas required for the process is combined with the slurry feed being fed into the bottom of the hydrocracking zone. About 10 to 35% by volume of the total hydrogen-containing gas being fed into the hydrocracking zone is fed in as the secondary hydrogen-containing gas with the slurry feed at the bottom of the reactor. The purpose of adding the hydrogen-containing gas to the slurry feed is to increase the flow velocity, to control coking in a fired liquid heater and to sweep the bottom of the reactor. Introduction of the relatively cooler slurry feed plus secondary hydrogen-containing gas keeps the bottom head cooler to prevent coking reactions and keeps it free of particles which could settle out from the reaction mixture. To achieve this, it has been found that the combined liquid plus gas velocity should be at least 5 m/s at the point of entry. Addition of liquid alone does not create sufficient turbulence.
In order to achieve the desired sweeping effect on the bottom of the reactor, the combined slurry feed and secondary hydrogen-containing gas is preferably fed into the reactor through an injector having a plurality of side openings which direct flow in an outward direction. During upset conditions, or if the reactor is in the coking mode, then mesophase particles can grow and fall through the reactor to the bottom head, where they accumulate. This mesophase is mixed and cooled by the incoming liquid plus gas feed, and can be maintained without coking problems for several hours. At some point it can be removed by dragging. Additionally, in conditions described above, feed flow can be increased, temperature decreased; and cold gas flow can be increased as well to resuspend stubborn solids and mesophase to facilitate dragging and recovery.
A further aspect of the present invention is an apparatus for carrying out the above hydrocracking process. The apparatus includes a vertical, elongated, cylindrical pressure vessel with a generally dome-shaped bottom head. This bottom head includes a feed injector adapted to feed a mixture of feed slurry and hydrogen-containing gas into the bottom of the vessel in an outward and upward direction. A higher circular array of nozzles is positioned adjacent the outer wall of the vessel in the region of the bottom end of the cylindrical portion and conduit means are provided for feeding a hydrogen-containing gas through these nozzles. A lower, axial circular array of nozzles is positioned within the dome-shaped bottom head and having a diameter less than one-half the diameter of the vessel. Conduit means are provided for feeding a hydrogen-containing gas through these nozzles. A reaction product outlet is provided in the top of the vessel. The feed injector and the gas inlet nozzles are arranged to move the content of the vessel upwardly through the vessel in a substantially plug flow with a minimum of settling or channelling.